1. Field of the Invention
This invention relates in general to acoustic surface wave filters which comprises a transit time delay arrangement for a wave passing in one direction in the filter or can also comprise a resonator with an acoustic wave standing in the resonator and the invention can be applied to either of these two different filter arrangements.
2. Description of the Prior Art
The publication "Electronics Letters", Volume 9, (1973), Pages 195-197 illustrates in FIG. 2 a 98 MHz oscillator which utilizes a filter of the type to which this invention relates. This filter utilizes a relatively wide band interdigital structure which has a length of r.multidot..lambda. and an interdigital structure which has a narrow band which has a length of p.multidot..lambda.. The narrow band interdigital structure is not completely filled over its length with digital strips or fingers as this would not be necessary for the narrow band condition but this elimination of certain of the fingers increases the insertion loss of the filter. Both of the interdigital structures of the prior art have the same predetermined mean or center frequency f.sub.0. The reason for not completely filling out a converter or transducer interdigital structure with digital strips, is to decrease the signal interferences (distortion signal through reflections to an interdigital structure of the filter. Such reflections are caused, for example, by waves which pass from the narrow band structure having a length of p.multidot..lambda. in the reverse direction due to reflection which waves impinge upon the wide band structure having a length r.multidot..lambda. and are reflected to the digital strips of such structure again and then proceed as time delay signals to the narrow band structure. A signal thus occurring at the narrow band structure is designated as a "triple transit" signal. The occurrence of such interfering or distortion signals causes a so-called "ripple" in the amplitude and frequency response F.sub.0 ;.phi..sub.0 of the filter.
In addition to the use of digital structures in which some of the fingers are missing, other methods have been proposed so as to more or less suppress the triple transit signal occurring because of the undesired multiple reflections caused by the interdigital structures. For example, the digital strips have been divided in one structure into two parallel strips connected with one another electrically with in each case having fingers which are only half as large as the width of the conventional fingers. Since, however, the maximum admissable width of a digital strip or finger is determined by the desirable frequency or particularly the mean frequency f.sub.0 of the filter, the use of "split fingers" results in extreme requirements with respect to the manufacture of extremely narrow digital strips and also decreases the maximum useable frequency by a factor of 2 in a filter so constructed.